1. Field of the Invention
The present invention relates to a fuel injection control apparatus for a diesel engine in which the fuel injection amount of a fuel injection pump thereof is controlled by an electrical control circuit.
2. Description of the Prior Art
Heretofore, as well known, the injection amount of a fuel injection pump has been controlled in such a manner that a ring member for opening and closing a fuel overflow port is moved in the axial direction of a plunger by means of a mechanical governor which is operated by virtue of a centrifugal force of a flyweight or the like, to change the effective pressure stroke of the plunger.
Recently, also in such type of diesel engine, a well controllable fuel injection pump utilizing an electronic control has been developed in an attempt to improve fuel economy and the purification of exhaust gas. However, in order to directly follow the above conventional construction and control the position of the ring member accurately, it is necessary to use a highly accurate actuator and position sensor, thus inevitably resulting in a complicated construction and an increased cost.
In Japanese Patent Publication No. 39854/1972, there is disclosed an arrangement wherein the fuel injection amount is controlled not by a ring member of a fuel injection pump but by a solenoid valve disposed in an intermediate position of a high pressure passage which provides communication between a fuel pressurizing chamber and an injection nozzle. The solenoid valve is opened to inject fuel at an appropriate time point during the period during which a fuel pressurizing pump (plunger pump) pressurizes fuel, and it is closed at an appropriate time point to stop the fuel injection. For the crank shaft of an engine, the injection time point is controlled by the injection start timing and the injection amount is controlled by the period until the stopping of injection. However, in the application of such control unit to vehicles, a wide variable range of fuel injection amount is required from the aspects of exhaust gas, fuel consumption and driving performance, and in order to cover the entire range, it is necessary to prolong the fuel pressurizing period. In the case of a plunger pump, prolongation of the pressure stroke is unavoidable. But, if the pressure stroke of the plunger pump is made long, a restriction is placed on a cam mechanism which is for changing a rotational motion into a linear motion, thus causing problems such as, for example, an abnormal lift of the plunger at a high revolution and a deficiency in mechanical strength.